The present invention relates generally to improved methods for perforating wells, such as oil and gas wells, and particularly to methods for generally aligning perforations along the direction relative to horizontal stress in the formation surrounding the wellbore.
In the completion of oil and gas wells, it is common to utilize multiple charge perforating guns to perforate the well casing and the formation surround the wellbore within a zone of interest. Such perforations may be directed toward one side of the wellbore or another. For example, see U.S. Pat. No. 4,194,577 issued Mar. 25, 1980 to Roy R. Van, and entitled "Method and Apparatus for Completing a Slanted Wellbore." U.S. Pat. No. 4,194,577 discloses orienting a perforating sun through gravity to perforate the low side of a non-vertical wellbore. Techniques have been proposed in co-pending parent application Ser. No. 897,538, filed Jun. 11, 1992, in the names of some of the inventors named herein, for determining the direction of stress fields within a formation, and for orienting perforations relative to those stress fields to promote efficient, subsequent, hydraulic fracturing of the formation. The orientation apparatus is described also in application Ser. No. 897,257, filed Jun. 11, 1992, by James J. Venditto, David McMechan, Calvin Kessler, and Harold E. Peelman.
Some formations, however, such as those conventionally referred to as "loosely consolidated" or "unconsolidated" formations, often present problems in the production of sand or larger formation pieces. A relatively high production rate from a relatively loosely consolidated formation typically results in a relatively increased pressure drawdown across the formation proximate the wellbore (i.e., in the "near wellbore area"). This pressure drawdown places increased stress on the formation. Where this stress (coupled with the pre-existing in situ stress) exceed s the shear strength on the formation, failure of the formation will typically occur, leading to sand production from the well.
In such circumstances, conventional practice is to install some type of sand control apparatus within the well. This may include merely placing a prepacked gravel pack screen within the well to minimize solids production into the wellbore. Production through such a pre-packed screen, however, may often become reduced over time due to collapsing and compacting of the formation around the screen.
Another, more expensive, remedy is to gravel pack the perforated zone by placing a volume of gravel in the formation, and actually in the perforations surrounding the well, to maintain the perforations in an open condition. Many techniques for gravel packing are well known to the industry. In general, however, gravel packing a well adds substantial additional time and expense to the process of completing the well. As a result, in many cases the decision as to whether or not to gravel pack a well will be based upon factors including how the unconsolidation of the formation, the resulting sand production and other disadvantages associated therewith, may be balanced against the cost of the gravel pack. At least in some cases, wells could be completed more efficiently if it were possible to minimize sand production from a well in a loosely consolidated or unconsolidated formation.
Accordingly, the present invention provides a new method and apparatus for determining the stress fields within the formation in the zone of interest surrounding a wellbore and for orienting the perforating gun to orient the perforations relative to the determined stress fields to thereby minimize sand production from the well. This method and apparatus have the further advantages that should the well be subsequently fractured, where the perforations are oriented generally in line with the maximum horizontal stress within the formation, the perforation tunnel will retain maximum stability; and any subsequent hydraulic fracturing operations should result in a maximum near-wellbore fracture width, and a desirable single fracture of maximum dimension.